This invention relates to a procedure for operating an uninterruptible power supply of a.c. loads which monitors a source of a.c. voltage, stores energy, and in the event of a reduction of the source a.c. voltage, supplies alternating current from the stored energy through an inverter. The invention further relates to a device to carry out this procedure.
Many loads, particularly data processing devices, require a.c. voltage supplies whose frequency and amplitude can vary only within very limited tolerances. Even short-term interruptions of data processing devices will result in operating malfunctions. In the event of a power failure of the normal a.c. power supply provided by a power company, the voltage supply required for sensitive loads must be drawn from a charged energy storage unit, such as a battery, and converted by a static converter into the frequency and amplitude required to supply the load units; at the same time any return flow of the energy drawn from the storage unit back into the malfunctioning power supply network must be prevented.
For that reason standard uninteruptible power supply systems often contain a rectifier connected to the a.c. power supply network, which as long as the a.c. power supply network is operating properly, keeps the energy storage unit in a fully charged status. An inverter connected to the uninterruptible bus supplying the load units at the output of the energy storage unit supplies the uninterruptible voltage which is drawn from either the a.c. power supply network and then rectified or, in the event of a power failure the voltage is drawn from the energy storage unit.
In this known arrangement, no switchover procedures are required, but the rectifying and subsequent inversion of the supply voltage at all times during the proper function of the a.c. power supply network, however, produces a substantial energy loss. This loss is often manifested by the generation of undesired noise and heat.
Another known possibility is to supply the load units directly from the power supply system, as long as the latter is operating properly, with the a.c. to d.c. and d.c. to a.c. converter arrangement required only to keep the energy storage unit charged. In the event of a power failure, however, the converter unit must begin feeding the load units immediately. Assuming that the energy storage unit is connected to the uninterruptible bus by means of a converter enhanced for reversible operation, then by means of a monitoring unit which monitors the status of the a.c. power system, the energy consumption of the load units and the load status of the battery, a control function can be set up so that the energy storage unit is kept in charged condition as long as the a.c. power network functions properly, with the converter practically functioning in no-load operation. U.S. Pat. No. 4,020,360 describes an inverter whose power component, moreover, can be shut down and held ready to start given a properly functioning a.c. power network and a charged energy storage unit, and is capable of commutating immediately in case of a power failure in order to draw the required power from the energy storage unit. Thus, during normal operation no energy losses result due to energy conversion in the power component.
In order to maintain the voltage at the uninterruptible bus without either phase or amplitude shifts, it is, however, necessary that in the event of a power failure the control of the inverter arrangement begin operating immediately with the voltage control and phase position prevailing at that moment. For that purpose, for example, in accordance with U.S. Pat. No. 4,020,360, an inverter model can be designed which supplies such respective ignition pulses--depending upon the load status of the energy storage unit, the phase position of the load voltage as well as the instantaneously required load unit current--that, given a functional power component, would enable the converter to supply the required capacity in the proper phase to the energy storage unit, but with these ignition pulses remaining inhibited due to the shut-down power component of the inverter. Only in the event of a malfunction of the a.c. power supply network will ignition pulses be connected to the simultaneously started power component.
With this standby operation of the inverter or of its control arrangement, the uninterruptible bus is separated from the a.c. power supply network by means of a switch as soon as a power supply malfunction occurs and there is the danger of feedthrough into the malfunctioning network from the uninterruptible bus which is now supplied out of the energy storage unit. The resulting switchover times have to be very short. It is preferrable if in this arrangement a filter connected to the uninterruptible bus is installed at or before the input of the load units, as in U.S. Pat. No. 3,999,078, which permits a buffering of the supply voltage as a short-term energy storage unit during the switchover time cycles. Said filter is also preferable in order to suppress voltage peaks and harmonic oscillations which are present in the a.c. power supply network or generated by the static converter operation.
The switch required to decouple the uninterruptible bus from the a.c. power supply network in the event of a system short-circuit is generally designed as an electronic switch in order the short switching times required. The current-carrying capacity of said electronic switches is, however, limited, i.e., in case of a short-circuit generated by the load units on the unterruptible bus, any surging short-circuit current which could result in a disruption of the electronic switch is not permissible. Thus, for example, a transformer is connected to the output of the electronic switch, which has two primary windings: the first one supplied from the a.c. power supply network over the switch, and the second one connected to the battery by means of a static converter. On the one hand said transformer limits the current flow from the a.c. power supply network to the interruptible bus, and on the other hand it permits a parallel supply of inductive-reactive current from the battery in the event that the system voltage, through present, is inadequate (or for that matter to reduce transitory system overvoltages by taking up inductive-reactive current), as well as, operation of the inverter as a voltage stabilizer.
The loads connected to the uninterruptible bus are generally protected by their own fuses so that, in the event of a malfunction, said loads do not convey a high short-circuit current onto the uninterruptible bus, thus leading to the breakdown of other loads similarly connected to the uninterruptible bus. Such fuses for the load units clearly have to be carefully designed with high sensitivity in accordance with the restrictive current-carrying capacity of the electronic switches utilized, or else they would induce high current levels to flow through the electronic switch, which would necessitate corresponding larger current capacity design features and elaborate arrangements of the electronic switch.